Prior art has described the photoprotective benefits of melanin-based light filters (Gallas, U.S. Pat. Nos. 4,698,374; 5,112,883; 5,036,115; 5,047,447) and methods for incorporation of melanin into various plastics.
Melanin is the pigment, as defined in the Gallas patents, and is traditionally synthesized by dissolving a suitable precursor, broadly a biological phenol, in water, incorporating an oxidizing agent—generally oxygen—and adjusting the pH to a sufficiently high value so as to allow polymerization to proceed. Under a free radical, autoxidation process, melanin is produced as a black, optically clear suspension. Under these conditions, particle sizes are approximately 50 A in size and such suspensions are optically clear, without any objectional haze or light scatter. Such suspensions are referred to in this application as either aqueous melanin or traditional melanin.
In most of the prior art involving applications of melanin in plastic light filters, it was necessary to incorporate melanin into optically clear, plastic substrates. This has been done by dispersing melanin powder into either liquid plastic monomer, followed by a curing process to harden the plastic, that is, in a thermoset process; or, melanin was dispersed in an extrusion process in which plastic, already formed was caused to flow and function as a solvent for the melanin, that is, in a thermoplastic process.
An essential point of this invention is that, in both cases above, it was necessary to first chemically modify, or derivatize, the aqueous melanin in order to disperse it so uniformly as to reduce any light scatter or haze to levels that would be acceptable in ophthalmic applications such as sunglass lenses. More specifically, aqueous or traditional melanin is an inherently hydrophilic polymer when synthesized in the traditional manner described above; and this makes it impossible to achieve a level of dispersion in liquid plastic or liquid plastic resin that results in adequately low haze for optical applications. For this reason, it is necessary to take strong and costly measures to chemically modify the polymeric surface groups in melanin, that, without such modification, render it hydrophilic, when trying to disperse it in plastics.
Another disadvantage to the prior art involving chemically modified melanins is that the derivatization process changes the optical spectrum of the melanin. Generally, the result is a bleaching of the spectrum with a loss in the optical density in the red part of the spectrum. This bleaching becomes further intensified during a thermoset process where the use of oxidizing curing agents causes still more bleaching of the melanin. Normally this problem is minimized by carefully choosing a suitable dye to restore the loss in the optical density in the red part of the spectrum. And in the case of a thermoplastic process, where an optically clear plastic is compounded with the derivatized melanin powder and extruded under high temperatures—greater than 200 degrees Celsius—and pressures, there is a frequent tendency for haze to develop in the plastic—possibly because of degradation of the melanin, producing low molecular weight volatiles or other degrading chemical reactions at such high temperatures.
Finally, there is a reduced solubility of even derivatized melanin in various plastics, which limits the concentration of the melanin and hence the darkness of the final filter because reduced solubility means unacceptable haze.